1. Field of the Invention
This invention relates to mobile telephony and, more particularly, to demodulation of text telephone data received via a mobile telephone medium.
2. Description of the Related Art
Text Telephone (referred to as TTY in North America) equipment has been used in the fixed telephone network for many years to transmit text and speech through the conventional telephone network. In such systems, a conventional telephone is connected to a terminal and the user enters the characters via a keyboard. Some types of TTY systems the telephone handset may be acoustically coupled to the terminal, while in other types of TTY systems the telephone may be directly connected. The terminal encodes the characters into tones (using a mapping code) which are then transmitted from the terminal through the normal voice path of the conventional telephone. A TTY terminal, in conjunction with a telephone on the receiver side, may decode the tones and display the characters on the terminal display.
In the United States, some cellular telephony systems support TTY services using a Baudot Code to map the characters using two tones: 1400 Hz and 1800 Hz. However, conventional digital cellular systems may not provide satisfactory character error rates for text transmitted in the speech channel using the traditional TTY modulation techniques developed for the fixed network. Thus, a new standard has been created to provide this cellular TTY service. The new standard is commonly referred to as Cellular Text Telephone Modem (CTM). During a call, the received signal may become so distorted or unreliable that it may not be decoded properly by the receiver.